Ground fault current interrupter

ABSTRACT

A differential amplifier circuit is connected between the secondary of a differential transformer and an electronic switch connected in series with a relay coil adapted to operate circuit breaker contacts when the current flow through the coil is changed. The differential amplifier circuit will sense a current unbalance in the transformer during either a negative or a positive half cycle and thereby immediately provide an output &#39;&#39;&#39;&#39;trip&#39;&#39;&#39;&#39; signal to the electronic switch. A delay circuit is connected between the differential amplifier circuit and the electronic switch to prevent accidental operation of the electronic switch by transient signals which are picked up by any part of the control circuit.



1. In a ground fault current interrupter including a control circuit which is adapted to detect a current unbalance between the current flowing in a first conductor connected between an alternating current source and a load and the current flowing in a second conductor connected between said source and said load and then to actuate open circuiting means connecting said conductors to said source, the improvement comprising, said control circuit including a differential transformer having a single magnetic core, two primary windings each of which is connected in series with one of said conductors, and a secondary winding, said primary windings being arranged so that under normal current conditions the total magnetomotive force produced in said magnetic core is balanced such that the net magnetic flux in said core is zero and under a current unbalance condition a net flux results which produces a voltage signal in said secondary winding of said differential transformer, amplifier circuit means connected to said secondary winding for developing a differential signal from the latter and for amplifying such signal, which amplifier circuit means includes means immunizing the latter from erroneous input noise signals aNd input transient signals, and means connected between said open circuiting means and said amplifier circuit means for actuating said open circuiting means upon receiving an amplified signal from said amplifier circuit means when said current unbalance is at or above a predetermined value for a predetermined time period.
 2. The improvement according to claim 1 wherein a resistor is connected in parallel with each of said primary windings and one of said resistors is a variable resistor whereby the magnetomotive forces developed in each primary winding can be made equal for a balanced current.
 3. The improvement according to claim 1 wherein said open circuiting means includes an armature associated with breaker contacts and adapted to be moved to cause opening of said contacts, a coil associated with said armature and adapted to cause movement of said armature upon a change in current flow through said coil, and an electronic switch connected in series with said coil to a source of direct current, said electronic switch also being connected to said amplifier circuit means so that an output signal from the latter causes a change in the current flow through said electronic switch to change the current flow through said coil and thereby cause opening of said breaker contacts.
 4. The improvement according to claim 1 wherein a signal delay circuit is connected between said amplifier circuit means and said open circuiting means to delay the application of the output signal from the amplifier circuit means to the open circuiting means and thereby provide protection against accidental actuation of said open circuiting means by transient signals.
 5. The improvement according to claim 3 wherein said open circuiting means includes said first-mentioned electronic switch and another electronic switch, said another electronic switch being connected between said amplifier circuit means and said first-mentioned electronic switch, said second electronic switch having high sensitivity to signals from said amplifier circuit means and said first-mentioned electronic switch connected in series with said coil being substantially insensitive to high rate of voltage rise turn on.
 6. The improvement according to claim 1 including a Zener diode connected between said amplifier circuit means and said open circuiting means and adapted to pass output signals from said amplifier circuit means which are above a certain value.
 7. The improvement according to claim 1 further defined by, said control circuit including other means adapting such circuit to be responsive to a current unbalance condition on both half cycles of the AC source.
 8. The improvement according to claim 7 wherein said other means include at least a pair of unidirectional current-blocking elements.
 9. The improvement according to claim 1 further defined by, a pair of resistors connected in parallel with said primary windings, respectively, one of said resistors being a variable resistor, the respective values of said resistors being such that the other of said resistors causes an unbalance condition in said primary windings within the range of adjustment of said one resistor thereby permitting the latter to restore said primary windings to a balanced condition.
 10. In a ground fault current interrupter adapted to be interposed in a pair of current supply lines between a current source and a load, the combination which comprises, means operable to open circuit said supply lines, means for detecting current flow unbalance between said supply lines and producing a voltage signal representative thereof, a differential amplifier circuit for amplifying said voltage signal, said differential amplifier circuit including means rendering the same insensitive to erroneous input noise signals and input transient signals, and means responsive to said amplified voltage signal attaining or exceeding a predetermined value for a predetermined period of time for rendering said open circuiting means operative.
 11. IN a ground fault current interrupter including a control circuit which is adapted to detect a current unbalance between the current flowing in a first conductor connected between an alternating current source and a load and the current flowing in a second conductor connected between said source and said load and then to actuate said circuiting means connecting said conductors to said source, the improvement comprising means associated with said conductors and responsive to said current unbalance for developing a voltage signal representative of said current unbalance, amplifier circuitry connected to said voltage signal developing means for amplifying said voltage signal, means for immunizing said amplifier circuitry from erroneous input noise signals and input transient signals, and means responsive to said amplified voltage signal for actuating said open circuiting means when said current unbalance has attained or exceeded a predetermined value for a predetermined time period. 